Fatty acids of alkyl esters are commonly known as biodiesel, a beneficial alternative fuel source due to its biodegradability, non-toxicity, and low emission profiles as compared to conventional fuel sources. Fatty acid alkyl esters are produced by transesterification; a reaction between triglycerides (animal fat and/or vegetable oil) and alcohol to form esters and glycerol.
Transesterification processes for producing alkyl esters have been known in the art. Historically, triglycerides in fats and oils have been methylated or otherwise esterified in a two-step process using an acidic catalyst, such as is described in the U.S. Pat. No. 4,695,411 to Stern et al., U.S. Pat. No. 4,698,186 to Jeromin et al., and U.S. Pat. No. 4,164,506 to Kawahara et al. Such processes included a pre-transesterification step, where fats/oils were reacted with alcohol in the presence of an acidic catalyst, and a subsequent transesterification step, where an alkaline/alcohol blend was added for the reaction completion. Transesterification processes employing alkaline catalysts, such as U.S. Pat. No. 5,525,126 to Basu et al., U.S. Pat. No. 5,908,946 to Stern et al., and U.S. Pat. No. 6,538,146 to Turck, have been known in the art, as well.
The transesterification process is known to be accelerated by using either excess alcohol for the reaction mixture or by removal of the by-product, glycerol. For Example, U.S. Patent Publication No. 2003/0032826 to Hanna is directed towards a process for the production of fatty acid esters that includes a tubular reactor having several reaction zones in series, while fresh alcohol and possibly catalyst is introduced into each reaction zone. Once the reaction mixture of the '826 publication is withdrawn from the last reaction zone, it is treated for separating the alcohol, glycerol, and fatty acid alkyl esters. The flow of the mixture within the '826 publication reactor is characterized with a Reynolds number of at least 2100. There is also U.S. Pat. No. 6,489,496 to Barnhorst et al., which is directed towards transesterification of carboxylic acid esters with alcohols in the presence of an alkaline catalyst, where the reaction product is passed through centrifugal separation to remove the glycerol. The reaction product of the '496 process may be passed either through a second centrifugal separator or recycled several times through the reactor to increase the yield of alkyl esters.
Removal of glycerol, in addition to introduction of fresh alcohol feed into the reaction, has been known in the art as well. For example, U.S. Pat. No. 5,116,546 to Klok et al. is directed towards a process for producing fatty acid lower alkyl mono-esters which comprises a first transesterification step between triglycerides and alcohol, a separation step wherein the glycerol is removed from the mixture, and a second esterification step in which fresh catalyst and alcohol are added to the remaining mixture. The '546 patent requires a residence time of 10 minutes to several hours for the first transesterification reaction and 1 hour to several hours for the second transesterification reaction, while the separation step is static and requires settling time.
Another example is U.S. Pat. No. 5,514,820 to Assmann el al., which is directed towards a continuous two-stage process for the production of lower alkyl esters in the presence of a homogenous alkaline catalyst at temperatures of up to 100° C. and under pressures of up to 10 bar. The process described in the '820 patent includes two tubular reactors in series (with a residence time of about 1-10 minutes), removal of glycerol after the first reactor, and use of fresh alcohol and catalyst feeds into the second reactor. There is also U.S. Pat. No. 5,354,878 to Connemann et al., which is directed towards a continuous multistage production of lower alkyl esters in the presence of an alkaline catalyst at temperatures of up to 100° C. The '878 patent discloses the use of a premix of fatty acids, alcohol, and catalyst fed into the top pf a column-reactor such that the rate of flow through the column is lower than the sinking rate of glycerin, which is continuously/discontinuously withdrawn from the bottom of the column. The '878 patent discloses a process that includes three such reactors, whereas a stirring reactor and a separator are located at the exit of each column reactor. See also, Australian Patent No. PJ 1105/88 (1988). However, these patents require multiple reactors in series in order to continue the transesterification process after the glycerol removal.
It is likewise known in the art to utilize a centrifuge between sequential reactors which involves multiple vessels and controls.
Despite the plentiful art, there exists a need for an efficient, cost-effective transesterification apparatus and process that provides high yield and short residence time in a compact equipment design.